This invention relates to the manufacture of paper and cardboard products and, more particularly, is concerned with a method of manufacturing paper and cardboard products which have improved strength characteristics.
Paper and cardboard products are generally made by pouring an aqueous stock of cellulosic fibres on to a wire mesh screen formed from a metal or a synthetic plastics material, and removing the water by drainage and/or other means such as suction, pressing and thermal evaporation. The cellulosic fibres are generally derived from wood which has been mechanically and chemically treated to form a pulp of fibrillated fibres which, when deposited on the wire mesh screen, interlock to produce a web, thus forming a paper or cardboard product. Other sources of cellulosic fibres include sisal, esparto, hemp, jute, straw, bagasse, cotton linters and rags.
The addition of a white filler to the cellulosic fibres improves the opacity, whiteness and ink receptivity of paper or cardboard products which are formed from the fibres. The white filler is also cheaper than the cellulosic fibres and therefore replacing some of the cellulosic fibres with the white filler can result in a cheaper product. The white filler may be, for example, kaolin, calcium sulphate, calcium carbonate, talc, silica or a synthetic silicate. The particle size distribution of a filler has an effect on its properties: on the one hand a filler which contains a significant proportion of relatively coarse particles may contain hard mineral impurities such as quartz or feldspar which makes the paper or cardboard product containing such a filler abrasive with consequent wear of type face and printing machinery; and on the other hand a filler which contains a significant proportion of relatively fine particles, i.e. particles having an equivalent spherical diameter smaller than about 2 .mu.m, has the disadvantage that the strength of the paper or cardboard product incorporating such a filler is reduced and in addition, unless expensive retention aids are used, a proportion of the filler which is added to the stock of cellulosic fibres tends not to be retained in the web of fibres but escapes with the "white water". i.e. the water which drains through the web and through the mesh screen, thus creating the problem of recovering the mineral particles before the effluent water can be discharged. Many retention aids, including aluminium sulphate, mannogalactans, starch and starch derivatives, have been incorporated in the furnish of filler and cellulosic fibres with a view to binding the filler to the cellulosic fibres.